Wild type and Cftr null mice on two different genetic backgrounds were used in this study. One group was congenic on the C57Bl/6 (B6) background, as previously described [21]. The other group was on a mixed background of about 95% B6 and 5% 129/Sv (129). The mice on the mixed background originated as part of a recently published study [20] as follows. Mice carrying a targeted mutation of the gastrin gene on a mixed B6/129 background [51] were bred for eight generations onto the B6 background. The gastrin(+/-) mice were then crossed for six generations with Cftr(+/-) mice congenic on the B6 background. A genome scan at this point showed that the mice were about 95% B6 and 5% 129 (see below). These mice were bred to obtain mice wild type for both gastrin alleles and either Cftr homozygous wild type [Cftr(+/+)] or Cftr homozygous null [Cftr(-/-)].